1. Field of Invention
The present invention relates to a system for estimating the position of a device that emits a detectable signal and, more particularly, to an emitter position estimating system that accurately determines the position of an emitter using time-synchronized receivers that employ cost effective, low accuracy clocks.
2. Description of the Related Art
The need to determine the position of an emitter can arise due to many different reasons, in diverse environments, and under highly variable physical conditions. For example, in search and rescue operations, the need may arise to locate an intentionally transmitted beacon signal, or a radio transmission believed to originate from a party requiring assistance. In both civilian and military environments, it may be necessary to locate the source of intentional or unintentional transmissions that are causing interference with authorized transmissions. Furthermore, the ability to determine the source of unauthorized transmissions allows the position of hostile equipment and personnel to be identified. The need to locate emitters arises in vastly different physical environments ranging from deserts, mountainous regions, and dense forests, to urban environments, and even extends to physical environments undersea and in space. In addition, emitters to be located in these environments may be fixed or mobile, traveling slowly or at high speed.
Techniques are known for estimating the position of a communication device using direct communication via an established protocol. These techniques measure the range between devices and rely upon a precise determination of the signal propagation time between the devices. The signal propagation time can be derived by knowing the transmission and reception times of one or more ranging signals traveling along a direct path between the devices. For example, the well-known global positioning system (GPS) relies on measurement of the one-way propagation time of signals sent from each of a set of satellites to a receiving device in order to determine the range to each satellite and the position of the receiving device. Position estimating systems that rely upon a two-way, round-trip ranging signal scheme are described in U.S. patent application Ser. No. 09/365,702, filed Aug. 2, 1999, entitled “Method and Apparatus for Determining the Position of a Mobile Communication Device Using Low Accuracy Clocks” and U.S. patent application Ser. No. 09/777,625 filed Feb. 6, 2001, entitled “Methods and Apparatus for Determining the Position of a Mobile Communication Device”, the disclosures of which are incorporated herein by reference in their entireties. In the ranging schemes described in these applications, a master mobile communication device transmits outbound ranging signals to plural reference communication devices which respond by transmitting reply ranging signals that indicate the position of the reference radio and the signal turn around time (i.e., the time between reception of the outbound ranging signal and transmission of the reply ranging signal). Upon reception of the reply ranging signal, the master radio determines the signal propagation time, and hence range, by subtracting the turn around time and internal processing delays from the elapsed time between transmission of the outbound ranging signal and the time of arrival of the reply ranging signal. The accuracy of the position determined by these systems depends largely on the accuracy with which the receiving devices can determine the time of arrival of the ranging signals traveling along a direct path between the devices.
Where an emitter is not directly cooperating with devices attempting to determine the emitter's position, the task of estimating the position of an emitter device is complicated by the fact that the emitter must be located without the benefit of two way communications. Of the existing techniques for determining the position of a non-cooperating emitter, the simplest and most common is the use of azimuth angle or bearing angle to track the emitter and determine its position. Unfortunately, this technique is reliable and accurate only if the transmitter is fixed and the emitted signal is sustained over a sufficient period of time to be tracked. Another technique uses multiple antennas to determine angle and range information. A recent adaptation of this technique is to use an antenna array and beam steering instead of multiple single antennas. Although an antenna array provides an improvement over using multiple antennas, it requires a bulky phased array antenna which increases size and cost and decreases mobility. Other existing techniques use extensive post processing to reduce a wealth of data into position information; however, these techniques also involve bulky and cumbersome equipment and/or fail to provide real time solutions.
As a whole, existing approaches suffer from a multitude of deficiencies which include: the inability to detect a broad range of emitted signal types; the inability to adapt to changes and advancements in technology; and the inability to support the demands imposed by varying operational environments, such as the need to operate under extreme weather conditions and in highly obstructed, multipath environments. Existing techniques rely upon specialized equipment that cannot leverage off the existing base of commercially available hardware and software. Furthermore, existing solutions often require bulky equipment for post processing and/or multiple antennas or antenna arrays which result in systems with increased size, power and weight, thereby increasing costs, while limiting mobility and the environments in which they can be used. In addition, existing approaches, due to their inability to periodically synchronize to a common time frame of reference, require high accuracy clocks to meet operational requirements for position accuracy, thereby significantly increasing costs. Existing solutions often require significant post processing and are unable to provide a high degree of mobility as well as real-time position estimation capability.
A strong need exists for a physically compact emitter position estimating system that can detect a variety of emitted signal types, can provide highly accurate real time solutions, is highly mobile, can detect fixed and highly mobile emitters, and can operate in physically diverse, severely obstructed, multipath environments. Such an emitter position estimating system should be able to locate an emitter without tracking the emitter over time, and should not require use of multiple antennas, antenna arrays, or directed antenna. The system would preferably employ an approach that allows the use of less expensive, commercially available hardware and software, and not require extensive post processing. Furthermore, it would be desirable for the emitter position estimating capabilities to be transparently integrated within devices supporting multipurpose communication related capabilities, thereby allowing widespread deployment at minimal incremental cost.
An emitter position estimating system that can meet these needs would be of great benefit in a variety of applications such as locating sources of intentional or unintentional interference, locating criminal and/or hostile force equipment, and facilitating search and rescue missions. Furthermore, such an emitter position estimating system, can bemused as part of a high performance, cost effective tracking system. Such a tracking system would be less expensive to build and deploy, and the complexity of deployed emitter devices would be greatly reduced, due to relaxed performance requirements, thereby reducing size, power requirements, size, and cost of deployed emitter devices, while increasing compatibility with deployed emitters produced by different vendors. Such a tracking system can be used to locate any style of beacon device or emitter deployed.
More generally, such a tracking system can be used to track the position of personnel and resources located both indoors or outdoors, including but not limited to: police engaged in tactical operations; firefighters located near or within a burning building or forest fire; medical personnel and equipment in a medical facility or en route to an emergency scene, including doctors, nurses, paramedics and ambulances; and personnel involved in search and rescue operations. An accurate, efficient emitter position estimating system would enhance capabilities to track and locate high-value items, including such items as personal computers, laptop computers, portable electronic devices, luggage, briefcases, valuable inventory, and automobiles. In urban environments, where conventional position estimating systems have more difficulty operating, it would be desirable to more reliably track fleets of commercial or industrial vehicles, including trucks, buses and rental vehicles. Tracking of people carrying an emitter is also desirable in a number of contexts, including, but not limited to: children in a crowded environment such as a mall, amusement park or tourist attraction; location of personnel within a building; and location of prisoners in a detention facility. The capability to accurately and efficiently determine the position of an emitter also has application in locating the position of cellular telephones. The capability to determine the position of cellular telephones could be used to pinpoint the position from which an emergency call has been made. Such information could also be used to assist in cell network management (for example, by factoring each mobile communication device's position into message routing algorithms).